The epithelium of the lung of mammals is known to contain at least two principal classes of neuroendocrine-like, or small-granule cells, those which exist as solitary cells and those which exist in an organoid structure referred to as the neuroepithelial body. (1) In one study, quantitation of the solitary neuroendocrine cells of the cricoid epithelium of the guinea-pig larynx by electron microscopy revealed a homogeneous population consisting of 4-9% of the total epithelial cells. The neuroendocrine cells identified in the tracheal epithelium had the same ultrastructural properties as those observed in the cricoid. (2) Continued efforts to identify known polypeptide hormones in the trachea by immunocytochemical techniques revealed no positive cells. (3) Tracheal neuroendocrine cells, identified by argyrophilia in light microscopy, did not incorporate 3H-thymidine into cell nuclei until approximately 3 days after administration of tracer, as demonstrated by autoradiography. This finding supports the notion that lung neuroendocrine cells are derived from a separate cell population. (4) The hyperplasia of lung neuroendocrine cells observed in hamsters treated with diethylnitrosamine appears to originate with cells of the neuroepithelial body or its progenitor.